This invention relates to separation systems using vacuum filtering cells. More particularly, this invention relates to improvements in the distributor valve which is downstream of the vacuum filtering cells.
Vacuum filtering cells are used to filter industrial feed slurry and in the manufacture of phosphoric acid by the wet method. The distributor valve separates gases from liquids and distributes the filtrates to collecting tanks.
Continuous vacuum filtering cells comprise horizontal filtering cells mounted on a carousel or turntable for periodic charging, draining and washing. Feed slurry is fed into the upper surface of the horizontal cell which includes a filtering element to remove solids from the feed slurry. A liquid/gas filtrate passes through the filter element to the bottom of the filtering cell and thereafter, drains out of the filtering cell through a flexible hose into a central distributor valve. In addition, a vacuum, originating in the distributor valve and applied to the bottom of the cell through the flexible hoses, assists in drawing the liquid/gas filtrate through the filtering element toward the central distributor valve. The remaining solids in the filtering cell are vacuum dried and then removed by inverting the filtering cell.
The distributor valve is divided into chambers and compartments which are connected to vacuum and pressurizing sources, respectively. During operation, the filtering cells are cyclically connected to a primary vacuum source to draw the liquid/gas filtrate through the filtering element toward the distributor valve. Next, the filtering cells are cyclically connected to a secondary vacuum source to dry solids remaining in the filtering cells. The filtering cells are then inverted and cyclically connected to the pressuring source to aid removal of the solid filtration cake from the filtering cells. Finally, the cells are washed and prepared for recharging.
The distributor valve achieves separation of the liquid/gas filtrate by drawing the gas component of the filtrate horizontally toward the vacuum source. The liquid component of the filtrate, which is too heavy to be drawn horizontally, falls vertically into downlegs which are connected to seal tanks for collection of the separated liquid. In the conventional design, as shown in FIG. 1, separation chamber 70 is divided into an inner section 72 and an outer section 74 by a vertical arcuate wall 76 extending the full height of the separation chamber 70. The vertical wall 76 typically has small passageways 78 at the top to permit the gas component of the liquid/gas filtrate to enter the outer section 74. The gas component typically follows a path as shown by the arrow. In addition to the gas flowing between the inner section 72 and the outer section 74, however, a certain amount of vapor containing suspended liquid particulates also flows through the dividing wall 76 into the outer section 74.
During the separation process, scaling forms on the inside surfaces of the distributor, thereby reducing its efficiency and requiring frequent stoppages for cleaning. Scaling often occurs in the distributor as a result of the high velocity of the gas and vapor travelling through the small passageways 78 in the dividing wall 76. The high gas and vapor velocity is a result of the narrow dimensions of the separation chamber 70 and passageways 78. The dissolved particulates in the vapor, which are carried into the outer section 74 as a result of the high gas and vapor velocities also contribute to scaling.
Conventional distributor valves also give rise to a large pressure drop reducing their efficiency. Because the vertical wall 76 introduces a blocking effect, the amount of vacuum reaching the filtering cells is significantly less than the amount of vacuum in the outer section 74 of the separation chamber 70. More specifically, if a vacuum source providing a vacuum of twenty (20) inches of mercury is employed at the distributor valve, the resultant vacuum at the filtering cell may be only fifteen (15) inches of mercury.